It is well known that the "optical" approach to the problem of interconnecting different elements of a telecommunications systems or, more generally of data transmission systems, provides a solution to rate, cross-talk and impedance mismatching problems, which constitute limitations of interconnection techniques for electrical carriers.
The elements required to build an optical interconnection are the transmitter, the receiver and the transmission medium connecting them. Inside the transmitter there is a source of light radiation, made by a solid state device (a laser or a LED), which is associated with an electronic circuit arranged to control its operation. Transmission of the light emitted by the optical source is, in most cases, entrusted to an optical fiber, which must be accurately coupled with the emitting device in order to maximize the radiation collection efficiency.
Perfect coupling must be maintained throughout the operating time of the interconnection and therefore it must be accomplished with a technique that will guarantee its stability in the presence of thermal or mechanical disturbances. For this reason it is common practice to equip the transmitter with a length of optical fiber, called a pig tail, firmly secured so as to guarantee coupling optimisation and stability. The end of the pig tail is secured with the most suitable technique to the actual transport fiber.
Problems of impedance mismatching and of parasite couplings also affect the electrical connections inside the transmitter, between the driving circuit and the optical emitting device. These problems add to those of size, which must be kept to a minimum, and of dissipation of the power generated by the emitting device and by the driving circuit, which dissipation must be adequately controlled in order to guarantee the operating stability of the transmitter as a whole.
A known method of assembling the driving circuit and the laser (or LED) inside a transmission module is described in the article "IBM Enterprise Systems multimode fiber optic technology", by N. R. Aulet et al, IBM J. Res. Develop. Vol. 36, no. 4, pages 553-575, 4 Jul. 1992. According to this method, the integrated circuit driving the laser and the laser itself are housed on a common support, which may be a wafer, for instance made of ceramic material or silicon, and are connected to each other by means of a wire bonded at its ends on metal areas provided on the devices themselves for this purpose, according to the technique known as "wire bonding".
This solution does not allow reducing the distances of the connections below a certain measure and requires a total mounting area that is substantially larger than the area of just the integrated circuit containing the laser driving circuit. Moreover, aligning the emitting device with the optical fiber requires additional space for assembly operations, which are complex and laborious.